1. Field of the Invention
The present invention relates to a semiconductor power module having a switching semiconductor element and a control circuit for controlling it accommodated in the same package, and a power conversion device, such as an inverter including the semiconductor power module, and particularly to an improvement for enhancing the strength for overvoltage.
2. Description of the Background Art
A semiconductor power module has a main circuit having a switching semiconductor element, i.e., a semiconductor element for power control which performs switching operation and a control circuit having a controlling semiconductor element for controlling operation of the main circuit by exchanging signals with the main circuit incorporated in a single device. This semiconductor power module is mainly applied to an inverter for controlling operation of motors etc., or a power conversion device, such as an uninterruptible power supply.
In the semiconductor power modules, ones with high frequency which repeatedly cut off and connect power is required for a decrease in power loss, high speed response of an object of power control, such as a motor, enhancement of operational preciseness thereof, and so forth. Furthermore, the semiconductor power modules are demanded which can control larger power to be used to drive industrial large motors and the like. The insulated gate bipolar transistors (referred to as IGBT, hereinafter) are suitable for use as switching semiconductor elements in the semiconductor power modules because they have the advantages of being capable of high speed operation, being relatively easily made to have high breakdown voltage and large current capacity, and having high input resistance which facilitates voltage control.
FIG. 14 is a circuit diagram showing the circuit structure of a switching semiconductor element in a conventional semiconductor power module and the vicinity thereof. In this semiconductor power module, an IGBT 1 is used as a switching semiconductor element. The IGBT 1 is responsive to a voltage signal inputted to a gate electrode 5 to connect (ON) and cut off (OFF) a collector electrode 3 and an emitter electrode 4. Following this, the collector current (main current) intermittently flows from the collector electrode 3 to the emitter electrode 4.
A free wheel diode 2 is connected to the IGBT 1 in parallel. Further, a clamping circuit including a Zener diode 6, a diode 7 and a resistor 8 connected in series is connected between the collector electrode 3 and the gate electrode 5. The free wheel diode 2 serves to prevent the IGBT 1 from breaking down due to a reverse flow of load current when the IGBT 1 turns off from on because of the inductive load connected to the IGBT 1 in parallel. The clamping circuit serves to prevent application of excessive voltage to the IGBT 1 resulted from an abnormality occurring in the semiconductor power module itself, or in the power conversion device or the like including the semiconductor power module to protect the IGBT 1 from breakdown.
For example, in the power conversion device, an excessive voltage may be applied to the IGBT 1 if the IGBT 1 turns off when it is to turn on because an abnormal signal is inputted. Or, when the IGBT 1 is ON, if an excessive collector current flows to the IGBT 1 because of an abnormality, such as short of load, and an overcurrent protection circuit included in the semiconductor power module operates to cut off the excessive collector current, an excessive surge voltage may be generated in the IGBT 1. The clamping circuit controls such overvoltage below a certain voltage to protect the IGBT 1 from breakdown.
Conventional devices are also known which have a snubber circuit including a resistor, a diode and a capacitor instead of the clamping circuit to protect the IGBT 1 from the overvoltage.
In these conventional devices, first, in the semiconductor power module using the snubber circuit, there have been such problems as mentioned below. If the circuit constant is set supposing occurrence of an abnormality, it may unnecessarily operate for the surge voltage in normal switching operation. Or, the power loss in the snubber circuit itself can not be neglected in the normal operation, and the conversion efficiency of the power conversion device or the like decreases.
Furthermore, the semiconductor power module having the clamping circuit shown in FIG. 14 had such problems as described below.
First, the inductance component of the line connecting the semiconductor power module and the clamping circuit may delay the response of the clamping circuit, or, the surge voltage caused by the inductance component of the line may hinder sufficient protection of the IGBT included in the semiconductor power module.
Second, when the clamping circuit operate to control application of excessive voltage to the IGBT 1, the clamp current flowing in the clamping circuit may cause the clamping circuit to overheat to result in breakdown, or stable operation may not be ensured. This is especially serious in a circuit in which the IGBT 1 is turned off when the excessive collector current, or the overcurrent, flows to the IGBT 1 to prevent overheat and breakdown of the IGBT 1, i.e., in the semiconductor power module including the overcurrent protection circuit described above. That is to say, if an abnormality occurs and an overcurrent flows when the IGBT 1 is ON, an excessive surge voltage generated by the overcurrent protection circuit operating is controlled in the clamping circuit. At this time, the IGBT 1 is in the OFF state because the overcurrent protection circuit is operating, with the result that a large load current flows into the clamping circuit.
Third, in the power conversion device and the like, when an abnormality occurs accompanied by no overcurrent, such as when an excessive voltage is applied to the IGBT 1 because the IGBT 1 turns off when it is to turn on, the clamping circuit will continuously operate to cause overheat of the IGBT 1 and possibly lead the IGBT 1 to breakdown. In such a case, since the overcurrent protection circuit does not operate because there is no overcurrent, the clamp current is supplied to the gate electrode 5 when the clamping circuit operates, and the IGBT 1 is not in a perfect OFF state but it is in a half ON state. That is, with the collector voltage of the IGBT 1 (the voltage between the collector electrode 3 and the emitter electrode 4) maintained at the clamp current defined by the clamping circuit, the collector current, which should originally flow in the ON state, flows in the IGBT 1 to cause excessive power loss in the IGBT 1. This causes overheat of the IGBT 1, and further, causes breakdown of the IGBT 1.
Due to such problems, the IGBT 1 is required to have excessive breakdown voltage with respect to the rated voltage of the semiconductor power module or the power conversion device. Or, they cause a problem that the rated voltage in the semiconductor power module or the power conversion device including the IGBT 1 having a certain breakdown voltage can not be set to a sufficiently high value which corresponds to the breakdown voltage of the IGBT 1.
This is especially considerable in a three-level inverter which is a kind of power conversion device. FIG. 15 is a circuit diagram showing the circuit structure of a conventional three-level inverter. A circuit 21 for a single phase is depicted as a representative in FIG. 15. If the three-level inverter is a single-phase inverter, two of the circuits 21 are connected in parallel, and if it is a 3-phase inverter, three of the circuits 21 are connected in parallel.
In this device, as shown in FIG. 15, four stages of semiconductor power modules 22-25 are connected in series between a high potential side power-supply terminal P and a low potential side power-supply terminal N. These semiconductor power modules 22-25 include IGBTs 22a-25a and free wheel diodes 22b-25b, respectively. Direct current power-supply voltage Ed is applied between the high potential side power-supply terminal P and the low potential side power-supply terminal N from an external power source.
Two capacitors 26 and 27 having equal capacitance and connected in series are also interposed between the high potential side power-supply terminal P and the low potential side power-supply terminal N. Voltage dividing resistors (not shown) are connected in parallel to the capacitors 26 and 27 so that the potential at the connection (intermediate potential point) O thereof becomes the intermediate potential of the high potential side power-supply terminal P and the low potential side power-supply terminal N. That is to say, the capacitors 26 and 27 hold the DC voltage of Ed/2 corresponding to a half of the power-supply voltage Ed, respectively.
The connection between the two semiconductor power modules 22, 23 and the intermediate potential point O, and the connection between the two semiconductor power modules 24, 25 and the intermediate potential point O are connected through diodes 28, 27, respectively. The diode 28 is interposed so that the direction from the connection O to the semiconductor power modules 22 and 23 is its forward direction, and the other diode 27 is interposed so that the direction from the semiconductor power modules 24, 25 to the connection O is its forward direction.
This three-level inverter further includes a control device 30. This control device 30 is connected to each of the semiconductor power modules 22-25 to send input signals to each of the semiconductor power modules 22-25. In each semiconductor power module 22-25, each IGBT 22a-25a performs ON operation and OFF operation in response to these input signals.
FIG. 16 shows an operation description diagram showing operation in the normal time in this three-level inverter. FIG. 17 shows a timing chart of voltages at respective portions in the normal operation shown in FIG. 16. Shown in FIG. 17 are waveforms of a voltage VU-O of the output terminal U which is a connection of the two semiconductor power modules 23 and 24 in relation to the intermediate potential point O and collector voltages V22-V25 of the respective semiconductor power modules 22-25.
Referring to these figures, the normal operation of this three-level inverter will be described. In the normal operation, the three-level inverter sequentially repeats the three kinds of operation modes, mode 1 to mode 3. In the mode 1, the semiconductor power modules 22 and 23 turn on (ON) and other semiconductor power modules 24 and 25 turn off (OFF). Next, in the mode 2, the semiconductor power modules 23 and 24 turn on and other semiconductor power modules 22 and 25 turn off. Further, in the following mode 3, the semiconductor power modules 22 and 23 turn off and other semiconductor power modules 24 and 25 turn on.
The respective semiconductor power modules 22-25 operate in this way and the voltage VU-O becomes +Ed/2 in the mode 1, zero in the mode 2, and -Ed/2 in the mode 3. That is to say, the three-level inverter outputs voltages at three levels. In any of the modes, two of the semiconductor power modules 22-25 are turned off. Accordingly, the collector voltages V22-V25 applied to the semiconductor power modules 22-25 are always controlled to Ed/2 or below.
That is to say, since the two power modules 24 and 25 are OFF in the mode 1, the power-supply voltage Ed is divided equally to the two. As a result, the collector voltages V24 and 25 attain ED/2, respectively. In the mode 2, since the two semiconductor power modules 22 and 25 are OFF, the collector voltages V22 and V25 attain Ed/2, respectively. Further, since the two power modules 22 and 23 are OFF in the mode 3, the collector voltages V22 and V23 attain Ed/2, respectively.
As described above, as the collector voltages applied to the power modules used in the three-level inverter are always not more than 1/2 of the DC voltage Ed applied between the high potential side power-supply terminal P and the low potential side power-supply terminal N, it is satisfactory that each power module has the voltage blocking ability, or the breakdown voltage, being not less than 1/2 of the DC voltage Ed. That is to say, the three-level inverter has the advantage in being capable of handling inter-terminal voltage higher than the breakdown voltage of used power modules. Accordingly, the three-level inverter is usually used for inverters handling high voltage.
However, if a trouble of the control device 30 causes a situation out of the ON/OFF conditions in the mode 1 to mode 3, all of the power-supply voltage Ed may be applied to one of the power modules 22-25. As an example, if such an abnormality occurs in which the power modules 23-25 turn on and the remaining one semiconductor power module 22 only turns off, the power-source voltage Ed is applied to the power module 22. Then, if the breakdown voltage of the power module 22 is lower than the power-supply voltage Ed, the power module 22 will result in breakdown.
Also, when some of the semiconductor power modules 22-25 are ON, if other semiconductor power modules turn on, which are to turn off, due to an abnormal operation of the three-level inverter, excessive short-circuit current flows as collector current and the overcurrent protection circuit operates to cut off the excessive collector current, and then a high surge voltage resulted from the inductive component existing in the power-supply interconnection and the like may be applied to the semiconductor power module.
To protect the semiconductor power modules from such abnormal phenomena, the breakdown voltage of each semiconductor power module 22-25 must be set higher with respect to the power-supply voltage Ed. For example, the breakdown voltage of the IGBT included in the semiconductor power modules 22-25 used in a three-level inverter having its rated voltage of 1500 V had to be set to 2000 V. That is to say, conventional three-level inverters can not fully make use of its original good point.
Furthermore, self arc-suppression type elements, such as IGBT, have a tendency that the switching loss and the steady ON-loss increase if the breakdown voltage of the elements is set higher. Accordingly, when self arc-suppression type elements such as IGBT are used for the semiconductor power modules and the rated voltage of the semiconductor power modules is made high to enhance the breakdown voltage in the three-level inverter, such as the three-level inverter shown in FIG. 15, there has been a problem that the power conversion efficiency of the three-level inverter is degraded.